Biaxially oriented polymers have numerous advantages over unoriented polymers in that they possess higher strength, improved stiffness, increased toughness, and superior thermoformability. Biaxially oriented polymers have been successfully produced using techniques disclosed in Austen et al U.S. Pat. No. 4,282,277, the disclosure of which is incorporated herein by reference. Austen et al describe a process wherein a tubular product is initially produced by hydrostatic extrusion using a mandrel in combination with a conical die. The tubular product is then cut, flattened, and annealed utilizing expensive flattening and annealing equipment. While the mandrel-conical die approach results in excellent biaxial orientation of structures, it is necessary to expend considerable effort to remove residual curvature in the resulting sheet. Even when the curvature has been removed by reconfiguring the extruded tube into a flat sheet, residual and unbalanced stresses and strains in the original tube tend to subsequently reassert themselves, resulting in difficulties when using the sheet.
In order to avoid the expense of first forming a tube and subsequently slitting and flattening the tube to form a biaxially oriented sheet, a twin-belt process has been developed wherein lower pressures are required to produce biaxial orientation than comparable processes using stationary flat dies. In actual experiments, pressures of 7,000 psi (492 kg/cm.sup.2) were required for flat dies whereas pressures of only 800 psi (56.2 kg/cm.sup.2) were required for twin-belt machines. Because the flat die machines utilize higher extrusion pressures, they require much greater initial capital outlays than twin-belt apparatus.
While biaxially oriented sheet can be produced by platen forging and cross-rolling, each of these processes has the drawback of being a batch process as opposed to a continuous process and therefore has serious size and economic limitations. Consequently, it is difficult to produce elongated sheets of material. In addition, with sheet forged between parallel platens, the sheet must be produced from a circular blank in order to have uniform biaxial orientation. The blank must then be trimmed which adds an additional step and wastes material. While sheets produced by cross-rolling have a roughly rectangular shape, they exhibit wavy surfaces resulting from non-uniform elastic spring back of sheet emerging from the roll nip and consequently are not necessarily suitable for subsequent shaping and forming. Moreover, due to short deformation times in cross-rolling, the resulting elastic spring back leads to a reduction of desirable properties such as stiffness.
As set forth in the above-mentioned patent applications, 933,951; 268,405; and 277,815, biaxial orientation of thermoplastic sheet in continuous twin-belt processes requires at least three major steps. The first step is transport of thick polymer slabs by belts through an angled deformation zone where the slabs are squeezed so that the material flows both parallel and perpendicular to the extent of the belts and is biaxially oriented. Secondly, the biaxially oriented sheet is passed through a zone where the belts are parallel and wherein the molecular structure of the material is heat set or annealed. Finally, the sheet which has been annealed is transported through a cooling zone in order to reduce the temperature of the sheet to a level below its heat deflection temperature so that the sheet remains flat.
The twin-belt process makes an excellent product when used on semi-crystalline materials such as polyethylene, polypropylene, polybutylene, polyacetal, polyamide, polyethylene terephthalate, and polybutylene terephthalate. Twin-belt processing of amorphous polymers also provides significant improvements in strength, stiffness, and toughness. However, surface characteristics of amorphous polymers are difficult to control when run through an apparatus having steel belts. While smooth belt surfaces and belt lubrication provide some improvement in surface quality, the resulting product is still not commercially acceptable unless more drastic steps are taken to reduce sheet surface roughness. Twin-belt processing has heretofore been uneconomical for amorphous polymers because of the extra cost of surface treatment after the material is biaxially oriented.
Kataoka U.S. Pat. No. 4,629,650 describes a process for compression molding a thermoplastic resin. A removable skin layer is interposed between the surface of a mold or die and a thermoplastic core resin to be molded. An interface between the skin layer and mold or die is lubricated by dispersion of a liquid lubricant. The resin core is then compression molded at a temperature equal to or greater than the glass transition temperature. The patent discusses compression molding of polymethyl methacrylate and various other amorphous polymers with polyolefin and polyamide skin layers. Two claimed advantages are lower compression forces to produce biaxial orientation and improved surface smoothness. However, the patent does not mention twin-belt processing of amorphous polymers or a separate annealing step to improve surface quality.
It is a principal objective of the present invention to provide a process and apparatus for producing a biaxially oriented amorphous polymer product having commercially acceptable surface quality.
A related objective of the invention is to provide a process for biaxially orienting amorphous polymer materials that can be carried out continuously in a twin-belt apparatus.
Additional objects and advantages of the invention will become apparent to persons skilled in the art from the following detailed description.